What does it mean to "congratulate" someone? Basically, it means to tell someone "Congratulations". This is just a nice word that you say to a person when something great has happened to them. People congratulate each other on events like:

getting engaged

getting a job

winning a game

To express the reason for congratulating someone, use the phrase "congratulate (someone) on ___":

Aren't you going to congratulate him on winning?

Congratulations on your new house!